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Ch. 5
Cytoplasm
Figure 5.10
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Membranes are selectively permeable
– They control the flow of substances into and out of a cell
Membranes can hold teams of enzymes that function in metabolism
Phospholipid Bilayer 2 layers of
phospholipids Proteins
Transport Receptors Enzymes
Cholesterol Maintains fluidity
Copyright © 2001 Pearson Education, Inc. publishing Benjamin Cummings
Figure 5.11
In water, phospholipids form a stable bilayer
Figure 5.11B
Hydrophilicheads
Hydrophobictails
Water
Water
– The heads face outward and the tails face inward
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Membrane composed of different molecules Phospholipids Cholesterol Proteins
Phospholipid molecules form a flexible bilayer Cholesterol and protein molecules are
embedded in it Carbohydrates act as cell identification tags
Molecules move laterally
Because of polar and nonpolar regions of the phospholipid bilayer, the membrane allows certain materials in and certain materials out of the cell
Permits exchange of nutrients, waste products, oxygen, and inorganic ions.
Allows some substances to cross more easily than others:
Hydrophobic molecules—hydrocarbons, CO2, and O2 dissolve in and cross membrane
Very small polar molecules, including H2O can cross easily
Diffusion caused by the random movement of particles across a membrane
Movement due to concentration gradient Moving from a higher concentration to
a lower concentration No energy used
Movement continues until equilibrium reached Concentration is the same on both
sides of the membrane
A substance will diffuse from where it is more concentrated to where it is less concentrated
Only small, uncharged particles diffuse without assistance (CO2, and O2)
http://sussexhigh.nbed.nb.ca/swift/biology11/Cell%20Boundaries%20ONLINE.ppt#276,25,Section G
Diffusion of water across a selectively permeable membrane
Water diffuses down its own concentration gradient (from hypotonic solution to hypertonic solution)
Does not use energy
Hypertonic solution Solution contains more solute than the
solution it is compared with Hypotonic solution
Solution contains less solute than the solution it is compared with
Isotonic solution Solution contains same amount of solute
as the solution it is compared with
What kind of solution is inside the cell? Isotonic
Outside the cell? Isotonic
Which direction will water move? In and out at the
same rate
10 % NaCl
10 % NaCl
10 % NaCl
10 % NaCl
10 % NaCl
10 % NaCl
What kind of solution is inside the cell? Hypertonic
Outside the cell? Hypotonic
Which direction will water move? Into the cell
10 % NaCl
20 % NaCl
10 % NaCl
20 % NaCl
10 % NaCl
20 % NaCl
What kind of solution is inside the cell? Hypotonic
Outside the cell? Hypertonic
Which direction will water move? Out of the cell
30 % NaCl
10 % NaCl
30 % NaCl
10 % NaCl
30 % NaCl
10 % NaCl
Copyright © 2001 Pearson Education, Inc. publishing Benjamin Cummings
Water balance between cells and their surroundings is crucial to organisms
The control of water balance is osmoregulation
Aides transport of many polar molecules and ions that are inhibited by phospholipid bilayer (sugar, some hormones, ions such as K+ and Na+)
Pores—always open Gated channels—
open/close in response to stimuli
Solutemolecule
Transportprotein
Passive transport—no energy usedUses transport proteins embedded in the plasma membrane (ion channels)
http://sussexhigh.nbed.nb.ca/swift/biology11/Cell%20Boundaries%20ONLINE.ppt#276,25,Section G
Transport proteins share similar properties with enzymes:
They are specific for the solutes they transport
They can be saturated with solute—maximum rate occurs when all binding sites are occupied
They can be inhibited by molecules that resemble the solute (similar to competitive inhibition)
Substances are transported across a membrane, against the concentration gradient
Use carrier proteins embedded in the membrane
Use energy (ATP)
http://student.ccbcmd.edu/~gkaiser/biotutorials/eustruct/images/sppump.gif
Active transport in two solutes across a membrane
Figure 5.18
Transportprotein
1
FLUIDOUTSIDECELL
Firstsolute
First solute, inside cell, binds to protein
Phosphorylated transport protein
2 ATP transfers phosphate to protein
3 Protein releases solute outside cell
4 Second solute binds to protein
Second solute
5 Phosphate detaches from protein
6 Protein releases second solute into cell
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
1. 3 Na+ ions inside the cell bind to the pump. ATP donates a phosphate to the pump
2. The pump changes shape, transporting 3 Na+ across the membrane, and are released outside of the cell
3. 2 K+ ions outside the cell bind to the pump
4. The phosphate group is released and
5. 2 K+ ions are transported across the membrane
6. 2 K+ are released inside the cell
http://gotoknow.org/file/somluckv/Cell_membrane06.jpg
Substances that are too large for carrier proteins (proteins and polysaccharides)
Endocytosis—movement of substances into the cell (a.k.a. phagocytosis)
Exocytosis—movement of substance out of the cell
http://www.stanford.edu/group/Urchin/GIFS/exocyt.gif
Vesicle forms from a localized region of cell membrane that sinks inward; pinches off into cytoplasm
Vesicle usually budded from the ER or Golgi and migrates to cell membrane
Used by cells to incorporate extracellular substances
Used by secretory cells to export products (insulin in pancreas; neurotransmitter from neuron)
Process of importing macromolecules into a cell by forming vesicles derived from the cell membrane
Process of exporting macromolecules from a cell by fusion of vesicles with the cell membrane
EndocytosisExocytosis
Figure 5.19A
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Figure 5.19B
Phagocytosis—endocytosis of solid particles Forms food vacuoles that
fuse with lysosome to be digested
Pinocytosis—endocytosis of fluid droplets Takes in solutes dissolved
in the droplet
http://student.ccbcmd.edu/~gkaiser/biotutorials/eustruct/images/phagocyt.gif
http://student.ccbcmd.edu/~gkaiser/biotutorials/eustruct/images/pinocyt.gif
Imports specific macromolecules into the cell by inward budding of vesicles formed from coated pits Occurs in response to binding specific ligands to receptors on
cell’s surface Harmful levels of cholesterol can accumulate in
the blood if membranes lack cholesterol receptors
Figure 5.19
Three kinds of endocytosis
Figure 5.19C
Pseudopod of amoeba
Food being ingested
Plasma membrane
Material bound to receptor proteins
PIT
Cytoplasm
Protein that binds a specific signal molecule, allowing the cell to respond to the signal molecule
Some receptor proteins are attached to ion channels Changes permeability to a specific ion
Some may cause the formation of a second messenger, which acts as a signal molecule in the cytoplasm
Figure 5.20
Essential Biology with Physiology, 2nd ed., by Campbell, Reece, and Simon, ©2007. These images have been produced from the originals by permission of the publisher. These illustrations may not be reproduced in any format for any purpose without express written permission from the publisher.
BIOLOGY: CONCEPTS AND CONNECTIONS 4th Edition, by Campbell, Reece, Mitchell, and Taylor, ©2003. These images have been produced from the originals by permission of the publisher. These illustrations may not be reproduced in any format for any purpose without express written permission from the publisher.
BIOLOGY: CONCEPTS AND CONNECTIONS 4th Edition, by Campbell, Reece, Mitchell, and Taylor, ©2001. These images have been produced from the originals by permission of the publisher. These illustrations may not be reproduced in any format for any purpose without express written permission from the publisher.